As is well known to those skilled in the art, zeolite catalysts have been used in hydrocarbon processing. Prior art zeolites have been found to be effective as catalysts for cracking, hydrocracking, hydroisomerization, etc. Recent trends in resid hydroprocessing demand increased conversion of feed having a boiling point above 1000.degree. F. as well as improved hydrogenation selectivity as measured by Conradson carbon conversion and hydrodenitrogenation (HDN) of the 1000.degree. F.-products attained from cracking of the residue.
It is also known in the art to use certain zeolites in a fluid catalytic cracking process. The FCC octane barrel catalyst (i.e. a catalyst which permits attainment of both octane number and gasoline yield) typically contains ultrastable Y-zeolites or dealuminated Y-zeolites. The ultrastable Y-zeolite is generally obtained by hydrothermal or thermal treatment of the ammonium or hydrogen form of the Y-type zeolite at temperatures above 1000.degree. F. in the presence of steam. Ultrastabilization by hydrothermal treatment was first described by Maher and McDaniel in the U.S. Pat. No. 3,374,056. U.S. Pat. No. 3,449,070 to McDaniel et al. discloses a method of producing an ultrastable Y-zeolite by base exchanging a charge faujasite zeolite to reduce the alkali metal content. The Unit Cell Size of the product is 24.40 .ANG.-24.55 .ANG.. Ammonium exchange and a second hydrothermal treatment at a temperature of about 1300.degree. F. to 1900.degree. F. further reduces the Unit Cell Size down to 24.20 .ANG. to 24.45 .ANG.. Hydrothermal treatment removes tetrahedral aluminum from the framework but not from the zeolite cages or channels where it remains as a hydrated cation or an amorphous oxide.
The Silica to Alumina Ratio of fluid catalyst cracking (FCC) catalyst usually refers to the SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of the zeolite component. It differs from the atomic Si/A1 ratio by a factor of two. Framework silica/alumina and total silica/alumina mole ratios of the zeolite should also be distinguished. Only aluminum (or alumina) that is part of the zeolite crystal structure (the framework) is included in the former. Total or bulk silica/alumina mole ratio also includes the alumina or amorphous oxide debris left in the void spaces of the zeolite after dealumination.
A variety of methods of dealumination are known in the art. A reference which provides an informative overview of the various processes is CATALYTIC MATERIALS:RELATIONSHIP BETWEEN STRUCTURE AND REACTIVITY, Ed. White, T. E., et al., Chapter 10, American Chemical Society, Washington, D.C., 1984. Using a chelating agent such as EDTA, up to about 50% of aluminum can be removed from the zeolite in the form of a water soluble chelate, without any appreciable loss in zeolite crystallinity and about 80% of aluminum atoms can be removed from the zeolite in the form of a water soluble chelate while the zeolite maintains 60%-70% of its crystallinity. U.S. Pat. No. 4,503,023 teaches the procedure for treating zeolites with ammonium hexafluorosilicate. In this method, an aqueous solution of Na.sub.2 SiF.sub.6 is used to replace some aluminum atoms with silicon atoms in the zeolites. Silicon may replace up to 60% of the aluminum without crystal damage. Debris formation and structure defect are negligible in this process. Unit Cell Sizes range from 24.35 .ANG. to 24.60 .ANG..
In general, as the zeolite framework is dealuminated, the Unit Cell Size (A.sub.0) decreases. The number of aluminum atoms per unit cell (N.sub.A1) can be estimated from the Unit Cell Size according to the Breck-Flanigan relationship: EQU A.sub.o (in .ANG.)=24.191+0.00868 (N.sub.A1) [1]
Because the total number of aluminum and silicon atoms (N.sub.A1 +N.sub.Si) is constant, according to: EQU N.sub.A1 +N.sub.si =192 [2]
then, the framework Silica to Alumina mole ratio can be expressed as: EQU SiO.sub.2 /Al.sub.2 O.sub.3 =2 [(1.6704/UCS-24.19)-1)] [3]
where UCS is the Unit Cell Size in Angstroms equivalent to A.sub.0 (in Angstroms) of Equation 1.
X-ray diffraction Unit Cell Size measurements can therefore be used to estimate the framework Silica to Alumina Mole Ratios. The Unit Cell Size of the zeolite may also predict zeolite properties such as hydrothermal stability, total acidity, and acid strength.
The catalytic activity in a fluid catalyst cracking process and the octane number of the product gasoline also correlate with the Unit Cell Size of the zeolite catalyst component. Ritter et al. reported in NPRA Annual Meeting, Los Angeles; Mar. 23, 1986; paper AM-86-45, that in FCC pilot plant experiments, the Unit Cell Size correlated well with both the research octane number and motor octane number of product gasoline fractions. They reported that about 15% of the total octane gain is observed as the Unit Cell Size decreased from 24.45 .ANG. to 24.35 .ANG. , and an additional 35% of the octane gain occurred as the Unit Cell Size decreased from 24.34 .ANG. to 24.28 .ANG..
As is well known to those skilled in the art, FCC gasoline octane barrels can be improved if the Unit Cell Size equilibrates to approximately 24.30 .ANG.. Reducing the Unit Cell Size below 24.30 .ANG. increases gas yield but does not significantly improve octane. There is a continuing search for a narrow band of Unit Cell Sizes that will yield maximum octane barrels. In most of the prior art methods of dealuminating catalysts for cracking and hydrocracking processes, the Unit Cell Size is controlled to be in the range of 24.30 .ANG.-24.45 .ANG. in an attempt to achieve maximum activity for vacuum gas oil conversion in FCC operations.
Mass transfer within cracking and hydrocracking catalysts for heavy oil upgrading has a significant effect on gasoline and light gas oil selectivities. The cracking sites on or near the zeolite crystal exterior primarily crack feed molecules such as gas oils, vacuum gas oils and residua. The cracking sites in the zeolite crystal interior primarily crack smaller product molecules like gasoline. Improving access to the zeolite interior reduces gasoline recracking and enhances gasoline production. The most straightforward way to minimize diffusional limitation is to increase the secondary porosity of the zeolites and by reducing the zeolite crystal size via dealumination.
U.S. Pat. No. 3,506,400 to Eberly, Jr., et al., discloses treating a conventional faujasite having a silica/alumina mole ratio of 8-12 in the ammonia form with steam at 800.degree. F.-1500.degree. F. followed by acid treatment at temperatures and times effective to remove amorphous alumina. This reference discloses product zeolites with unit cell sizes greater than or equal to 24.28 Angstroms with alumina present and about 24.2 Angstroms for an alumina free crystalline polysilicate. This reference fails to disclose any product zeolite having a Unit Cell Size less than 24.19 .ANG.. This reference also does not teach how to increase the secondary pore volume of the zeolites in order to augment the cracking activity of heavy oils.
U.S. Pat. No. 4,840,930 to LaPierre et al. discloses treating a charge Y-zeolite to make it stable to acid. The charge is characterized by a silica/alumina ratio of 3-25. The charge is contacted with a steam-containing atmosphere at increasing temperatures in the 392.degree. F.-1202.degree. F. range. Specifically the critical rates of heating are (i)&gt;2.degree. and &lt;4.degree. C./min at 392.degree. F.-932.degree. F., and (ii) 0.2.degree.-0.5.degree. C./min at 932.degree. F.-1202.degree. F. These ranges are critical; and operating outside these ranges yields undesirable results. This reference does not teach a method of producing zeolite having a Unit Cell Size of 24.19 .ANG. or less or having an increased secondary porosity.
U.S. Pat. No. 4,512,961 discloses a process of producing a dealuminated Y-zeolite by a combination of hydrothermal treatment at a temperature from 932.degree. F. to 1652.degree. F. for a period of 1-5 hours and ion-exchange resin removal of aluminum from the crystalline structure of the zeolites at 212.degree. F. for a time about 1-4 hours. This patent does not reveal any product zeolite having a Unit Cell Size of 24.19 .ANG. or less.
U.S. Pat. No. 5,059,567 to Linsten et al., discloses a process for the preparation of a modified Y-zeolite having a unit cell size of 24.15-24.59 .ANG.. The product zeolite is produced by subjecting charge zeolite NaY to a series of treatment steps comprising ammonium ion exchange and calcination. If the unit cell size is to be reduced to 24.15-24.35 .ANG., an aluminum ion-exchange is carried out, followed by calcination in steam at 500.degree.-900.degree. C. The decrease in the unit cell size depends on the amount of aluminum supplied by ion-exchange. U.S. Pat. No. 5,242,677 to Cooper et al., discloses a method of preparing stable Y-zeolite with unit cell sizes of 24.09-24.14 .ANG. by the steps of aluminum ion-exchange, steam calcination, and acid treatment. The instant invention can be distinguished from these two references in that no aluminum ion-exchange step is required to produce a dealuminated Y-zeolite having unit cell sizes of 24.02-24.18 .ANG..
U.S. Pat. No. 5,243,121 to Madon et al., discloses a FCC catalyst comprising a non-zeolitic component of at least 45% Al.sub.2 O.sub.3 and containing no more than 30% Y-zeolite with unit cell sizes less than 24.29 .ANG.. As shown in Example 1 of the Madon, et al. patent, the hydrothermally treated Y-zeolites having unit cell sizes of 24.26 and 24.27 .ANG. gave greater yields of isobutylene than the zeolite having the unit cell size of 24.29 .ANG. and a steamed commercial Y-zeolite having the unit cell size of 24.23 .ANG..
We have now found that a dealuminated Y-zeolite having a Unit Cell Size smaller than about 24.19 .ANG. and increased secondary porosity can be reproducibly made by using the process of the instant invention. The resulting product zeolite is also characterized by presence of a significant quantity of non-zeolite components, i.e., amorphous silica-alumina oligomers. The product zeolite is useful in the conversion of heavy oils containing vacuum resids to lighter products, particularly light gas oil. The product zeolite is also useful in the FCC process for increased C.sub.3 -C.sub.5 olefin yields and reduced slurry oil yields as well as reduced coke-make. The product zeolite can be used as a catalyst for the conversion of propylene to isobutylene and isoamylene as well as for the conversion of paraffins to aromatics in the reforming process.
It is an object of this invention to provide an improved FCC process employing a zeolite having a lower unit cell size than previously possible. It is an object of this invention to provide a FCC process for increasing the yields of light olefins (C.sub.3 -C.sub.5) using a Y-zeolite treated by the novel process of copending Ser. No. 07/962,549 and (Ser. No. 08/206,803), said zeolite characterized by having a unit cell size below 24.19 .ANG. and as low as 24.02 .ANG.. Other objects will be apparent to those skilled in the art.